MTU problem only on router B - ospf/mpls/vpls based network

RouterOS Forwarding Protocols
1

Hi Guys,

this topic is for very experienced Mikrotik professionals or Mikrotik Staff !!

This is the scenario:
-2 CCR as core routers with 2 BGP upstreams and Full route table recived from both upstreams
-2 RB1100 AHx2 as PPPoE Servers and ospf routers with ospf area backbone to talk:

  1. with core routers (small core pubblic subnet) and dedicated ethernet interface
  2. with other routers (/30 private subnets), via dedicated ethernet interface
    -varius tower routers with RB750, RB1100 ecc. with ospf
    -on each tower we have also Access Points

On OSPF layer we make MPLS configuration and of course VPLS to gain L2 connectivity from Access Points: each AP is directly connected to an ethernet port of relative ospf tower router bridged with vpls interface to see PPPoE Server (each vpls is terminated on two RB1100 AHx2 as PPPoE Servers.

All MPLS interface used have MPLS MTU setted at 1526

From RB1100 AHx2 sides PPPoE Server are single bridge interface, each bridge contain only the relative vpls to each ospf tower router to reach Access Points.

Everything works fine with Router OS Ver 5.14, but customers are increasing and we want to obtain improvement on RB1100 AHx2 acting PPPoE Servers so we decide to upgrade Router OS on v 6.12 (same version we have on core BGP routers).

We decide to upgrade first RB1100 AHx2 with version 6.12 and everything was ok.

The day after we decide to make upgrade on the second RB1100 AHx2 (we don’t want differente Router OS version on our ospf routers) but
inexplicablement customers (pppoe client) served by the second RB1100 AHx2 PPPoE Server experimenting MTU problem: they can’t open some websites and, in the troubleshooting, i can’t ping with packet size 1480 the pppoe ip address with do not fragment option.

Someone can help to troubleshoot? We have already tryed to upgrade on last stable release but notinh changed.

Thanks in advacne and best regards.

2

Could you try to disable “use-ip-firewall” in bridge-settings of the mpls routers? I saw a strange behaviour in the past after upgrading to version 6.

3

PPPoE encapsulated in VPLS needs 1530 MPLS MTU…

Check page 18 and 19 of the presentation linked in my sig.

4

Is not enabled in our environment. By the way bridge in our environment are used only as container of vpls-interface coming from tower to avoid up/down problem on interface acting as pppoe_server. Using bridge_interface as pppoe-server interface instead of vpls interface.

Thanks for your reply.
Best regards,
Luciano

5

Hi Tomaskir,

In my environment i think 1526 of mpls-mtu is enough becouse I don’t have vlan.
By the way I have a look to your presentation and in parallel I am in contact with mikrotik support.
Remember that we have side 1 of infrastracture that work properly so I don’t understand why this problem happens!

Thanks for your reply.

Best regards,
Luciano

6

1526 is not enough, look at the presentation on the slides I linked.
You can see the vlan is outside the MPLS, so its not counted into the MPLS MTU.

Its entirelly possible your setup works, but your packets are getting fragmented.

There have been changes to packet fragmentation handling in newer versions, which is possibly what broke your setup.

7

Hi,
in respect of your slide 19 I have not consider 8 byte of PPPoE so I’m with you about correct value of MPLS MTU to 1530 instead 1526. But this not tell me why on side A everything works fine*.

*studing in deep I suppose you are right, on side A everything works but there is packet fragmentation.
But now I found a new mistake:
according to your slide I think I have a problem on MTU of the switch where L2 connection from Radio links arrive. Bypassing switch with mpls-mtu configured on 1530 on mpls routers everything works fine. But, considering that switch is the same for side A, why on side A works with fragmentation and on side B dose not work anything?

Thanks in advance and best regards.
Luciano